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Free water fasting calculator for 72-hour, 5-day, and 7-day water fasts: schedule dates, see estimated kg/lb loss (water + fat components), ketosis timing, and 2.5 L/day hydration. Default 3-day fast at 70 kg → 2.7 kg estimated. Not medical advice.
Last updated: May 25, 2026
Sustainable deficit instead? Fat loss calorie deficit
Fasting Duration
3 Days
(72 hours)
Estimated Weight Loss
2.7 kg
Estimated Fat Loss
0.6 kg
Hydration Recommendation
7.5 liters
Recommended total water intake during fast (3 liters/day average)
Calories Burned
4,610
Total
BMR
1618
cal/day
Metabolic Status
Likely in ketosis
Recommendations
We can build and embed a custom version of Water Fast Calculator for your brand and workflow.
Water Fasting Facts:
Default: 3-day fast · 70 kg · male · age 30
2.7 kg
~2.1 kg water + 0.6 kg fat
3 days
72 hours · Likely in ketosis
7.5 L
Total over the fast
70 kg · male · 30 yr · BMR 1618 kcal → ~2.7 kg loss · 7.5 L water
Days
3
Est. loss
2.7 kg
Fat (est.)
0.6 kg
Kcal burned
4,610
| Fast | Est. loss (70 kg male, 30 yr) | Fat / water split | Note |
|---|---|---|---|
| 72-hour fast (3 days) | 2.7 kg | 0.6 kg fat · 2.1 kg water | Ketosis common by hour 24–36 |
| 5-day water fast | 3.19 kg (75 kg, 35 yr) | 0.94 kg fat · 2.25 kg water | Higher total loss; monitor electrolytes |
| 7-day water fast | 3.35 kg | 1.25 kg fat · 2.1 kg water | Extended — medical guidance advised |
BMR (male) = 10×kg + 6.25×cm − 5×age + 5BMR (female) = 10×kg + 6.25×cm − 5×age − 161Daily fast burn = BMR × 0.95 (≤3 days) or × 0.85 (>3 days)Fat loss (kg) = (daily burn × days) ÷ 7700 · Water ≈ 3% body weight if ≤7 days| Window | Typical state |
|---|---|
| 0–12 hours | Glycogen stores still fueling brain/muscles |
| 12–24 hours | Entering ketosis — liver glycogen falling |
| 24+ hours | Likely in ketosis — fat oxidation rising |
| Day 3+ | Metabolic rate may drop ~10–15% (adaptive) |
BMR 1618 · burn ~1537 kcal/day × 3 = 4,610 kcal
Loss 2.7 kg (2.1 water + 0.6 fat)
BMR 1705 · ×0.85 → 3.19 kg (0.94 kg fat)
3.35 kg total · 17.5 L water · Likely in ketosis
Lower BMR → 2.44 kg est. (0.49 kg fat)
Share with anyone planning a structured water fast.
Suggested hashtags: #WaterFast #Fasting #Health #WeightLoss #Calculator